Color plus clear coating system utilizing inorganic microparticles

ABSTRACT

Disclosed is a method of coating a substrate comprising the steps of (A) coating the substrate with one or more applications of a basecoating composition containing (1) an organic film-forming resin, (2) substantially colorless, substantially inorganic microparticles stably dispersed in the basecoating composition, (3) a solvent system for the film-forming resin, and (4) pigment particles to form a basecoat; and (B) coating the basecoat with one or more applications of a topcoating composition containing (1) an organic film-forming resin, and (2) a solvent system for the organic film-forming resin of the topcoating composition, to form a transparent topcoat. The substantially inorganic microparticles prior to incorporation in the basecoating composition range in size from about 1 to about 150 nanometers.

This is a division of application Ser. No. 783,324 filed Oct. 2, 1985,which is a continuation of application Ser. No. 529,420 filed Sept. 6,1983, now abandoned.

BACKGROUND OF THE INVENTION

A coating system gaining wide acceptance, particularly in the automotiveindustry, is one which is known as "color plus clear". In this systemthe substrate is coated with one or more applications of a pigmentedbasecoating composition, which is in turn coated with one or moreapplications of a generally clear topcoating composition.

However, there are several difficulties in employing "color plus clear"coating systems especially as attempts are made to employ coatingcompositions having high solids contents and also as metallic flakepigments are used to provide a special two tone appearance to the coatedsubstrate as it is viewed from different angles to a direction normal tothe surface of the substrate. For example, it is important in a "colorplus clear" coating system that the applied basecoat not be attacked bycomponents of the topcoating composition, particularly solvents, at theinterface of the two, a phenomenon often referred to as strike-in.Strike-in adversely affects the final appearance properties of thecoated product. Strike-in is an especially serious problem whenmetallic-flake pigments are employed in the basecoating composition.Strike-in, among other things, can destroy the desired metallic-flakeorientation in the basecoat.

Additionally, irrespective of the problems associated with strike-in, itis important to prevent sagging during curing of the coating compositionafter application to a nonhorizontal substrate. Also, especially wheremetallic-flake pigments are employed, it is important to achieve andmaintain proper pigment orientation in the pigmented basecoatingcomposition during the curing or drying operation. Moreover, where amaterial is incorporated in the topcoating composition to preventsagging of the topcoating composition during cure, it is particularlydesirable that such material not seriously affect the clarity of thecured topcoat, for example, by imparting to the topcoat a cloudy ormilky appearance.

One attempt to address some of these problems has been to incorporate inthe basecoating composition as part of the organic polymer systempresent, a proportion of organic, insoluble polymer microparticles asdescribed for example in U.S. Pat. No. 4,220,679 to Backhouse. Anotherattempt to address at least some of the problems of achieving propermetallic-flake orientation in a high solids basecoat has been tosubstantially increase the amount of metallic-flake pigment in thebasecoating composition as described in U.S. Pat. No. 4,359,504 to Troy.

It has now been found that the incorporation of substantially inorganicmicroparticles in the basecoating composition permits the basecoatingcomposition to be formulated for example at high solids content andalleviates the problems of strike-in, the problems of achievingexcellent metallic-pattern control where metallic-flake pigments areemployed, and the problem of sagging of the coating composition on anonhorizontal substrate during curing or drying. It has also been foundthat the incorporation of substantially inorganic microparticles, forexample based on silica, in the topcoating composition, not onlyalleviates sagging of the topcoating composition during cure but alsodoes not seriously affect the clarity of the transparent topcoat.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a method for coating a substratecomprising the steps of: (A) coating the substrate with one or moreapplications of a basecoating composition comprising (1) an organicfilm-forming resin, and where the film-forming resin can be crosslinked,optionally a crosslinking agent for the film-forming resin, (2)substantially colorless, substantially inorganic microparticles stablydispersed in the basecoating composition, (3) a solvent system for thefilm-forming resin in which the inorganic microparticles do notdissolve, and (4) pigment particles, to form a basecoat; and (B) coatingthe basecoat with one or more applications of a topcoating compositioncomprising (1) an organic film-forming resin, which may be the same ordifferent from the film-forming resin of the basecoating composition,and where the film-forming resin of the topcoating composition can becrosslinked, optionally a crosslinking agent for the film-forming resinof the topcoating composition, and (2) a solvent system for the organicfilm-forming resin of the topcoating composition, to form a transparenttopcoat.

The present invention also provides a method for coating a substratecomprising the steps of: (A) coating the substrate with one or moreapplications of a basecoating composition comprising: (1) an organicfilm-forming resin, and where the film-forming resin can be crosslinked,optionally a crosslinking agent for the film-forming resin, (2) asolvent system for the film-forming resin of the basecoatingcomposition, (3) organic polymeric microparticles which are insoluble inthe solvent system of the basecoating composition and which have adiameter in the range of from about 0.01 to about 40 microns, and (4)pigment particles, to form a basecoat; and (B) coating the basecoat withone or more applications of a topcoating composition comprising: (1) anorganic film-forming resin which may be the same or different from thefilm-forming resin of the basecoating composition, and where thefilm-forming resin of the topcoating composition can be crosslinked,optionally a crosslinking agent for the film-forming resin of thetopcoating composition, (2) substantially colorless, substantiallyinorganic microparticles stably dispersed in the topcoating compositionranging in size from about 1 to about 150 nanometers, and (3) a solventsystem for the organic film-forming resin of the topcoating compositionto form a transparent topcoat.

DETAILED DESCRIPTION OF THE INVENTION

The film-forming resin of the basecoating composition may be any of thefilm-forming resins useful for coating compositions. The film-formingresins of the basecoating composition can be film-forming thermoplasticresins and/or thermosetting resins. Examples of such film-formingthermoplastic resins and/or thermosetting resins include the generallyknown cellulosics, acrylics, aminoplasts, urethanes, polyesters,epoxies, and polyamides. These resins, when desired, may also containfunctional groups characteristic of more than one class, as for example,polyester amides, uralkyds, urethane acrylates, urethane amideacrylates, etc. As indicated above, the film-forming resin may bethermoplastic or it may be thermosetting. As used herein, the termthermosetting is intended to include not only those resins capable ofbeing crosslinked upon application of heat but also those resins whichare capable of being crosslinked without the application of heat.

Cellulosics refer to the generally known thermoplastic polymers whichare derivatives of cellulose, examples of which include: nitrocellulose;organic esters and mixed esters of cellulose such as cellulose acetate,cellulose propionate, cellulose butyrate, and cellulose acetatebutyrate; and organic ethers of cellulose such as ethyl cellulose.

Acrylic resins refer to the generally known addition polymers andcopolymers of acrylic and methacrylic acids and their ester derivatives,acrylamide and methacrylamide, and acrylonitrile and methacrylonitrile.Examples of ester derivatives of acrylic and methacrylic acids includesuch alkyl acrylates and alkyl methacrylates as ethyl, methyl, propyl,butyl, hexyl, ethylhexyl and lauryl acrylates and methacrylates, as wellas similar esters, having up to about 20 carbon atoms in the alkylgroup. Also, hydroxyalkyl esters can readily be employed. Examples ofsuch hydroxyalkyl esters include 2-hydroxyethyl acrylate,2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropylmethacrylate, 3-hydroxypropyl-4-hydroxybutyl methacrylate, and mixturesof such esters having up to about 5 carbon atoms in the alkyl group. Insome instances, corresponding esters of other unsaturated acids, forexample, ethacrylic acid, crotonic acid, and other similar acids havingup to about 6 carbon atoms can be employed. Where desired, various otherethylenically unsaturated monomers can be utilized in the preparation ofacrylic resins examples of which include: vinyl aromatic hydrocarbonsoptionally bearing halo substituents such as styrene, alpha-methylstyrene, vinyl toluene, alpha-chlorostyrene, alpha-bromostyrene, andpara-fluorostyrene; nonaromatic monoolefinic and diolefinic hydrocarbonsoptionally bearing halo substituents such as isobutylene,2,3-dimethyl-1-hexene, 1,3-butadiene, chloroethylene, chlorobutadieneand the like; and esters of organic and inorganic acids such as vinylacetate, vinyl propionate, isopropenyl acetate, vinyl chloride, allylchloride, vinyl alpha-chloroacetate, dimethyl maleate and the like.

The above polymerizable monomers are mentioned as representative of theCH₂ ═C< containing monomers which may be employed; but essentially anycopolymerizable monomer can be used.

Aminoplast resins refer to the generally known condensation products ofan aldehyde with an amino- or amido-group containing substance examplesof which include the reaction products of formaldehyde, acetaldehyde,crotonaldehyde, benzaldehyde and mixtures thereof with urea, melamine,or benzoguanimine. Preferred aminoplast resins include the etherified(i.e., alkylated) products obtained from the reaction of alcohols andformaldehyde with urea, melamine, or benzoguanimine. Examples ofsuitable alcohols for preparing these etherified products include:methanol, ethanol, propanol, butanol, hexanol, benzylalcohol,cyclohexanol, 3-chloropropanol, and ethoxyethanol.

Urethane resins refer to the generally known thermosetting orthermoplastic urethane resins prepared from organic polyisocyanates andorganic compounds containing active hydrogen atoms as found for examplein hydroxyl, and amino moieties. Some examples of urethane resinstypically utilized in one-pack coating compositions include: theisocyanate-modified alkyd resins sometimes referred to as "uralkyds";the isocyanate-modified drying oils commonly referred to as "urethaneoils" which cure with a drier in the presence of oxygen in air; andisocyanate-terminated prepolymers typically prepared from an excess ofone or more organic polyisocyanates and one or more polyols including,for example, simple diols, triols and higher alcohols, polyester polyolsand polyether polyols. Some examples of systems based on urethane resinstypically utilized as two-pack coating compositions include an organicpolyisocyanate or isocyanate-terminated prepolymer (first pack) incombination with a substance (second pack) containing active hydrogen asin hydroxyl or amino groups along with a catalyst (e.g., an organotinsalt such as dibutyltin dilaurate or an organic amine such astriethylamine or 1,4-diazobicyclo-(2:2:2) octane). The activehydrogen-containing substance in the second pack typically is apolyester polyol, a polyether polyol, or an acrylic polyol known for usein such two-pack urethane resin systems. Many coating compositions basedon urethanes (and their preparation) are described extensively inChapter X Coatings, pages 453-607 of Polyurethanes: Chemistry andTechnology, Part II by H. Saunders and K. C. Frisch, IntersciencePublishers (N.Y., 1964).

Polyester resins are generally known and are prepared by conventionaltechniques utilizing polyhydric alcohols and polycarboxylic acids.Examples of suitable polyhydric alcohols include: ethylene glycol;propylene glycol; diethylene glycol; dipropylene glycol; butyleneglycol; glycerol; trimethylolpropane; pentaerythritol; sorbitol;1,6-hexanediol; 1,4-cyclohexanediol; 1,4-cyclohexanedimethanol;1,2-bis(hydroxyethyl)cyclohexane; and2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate. Examplesof suitable polycarboxylic acids include: phthalic acid; isophthalicacid; terephthalic acid; trimellitic acid; tetrahydrophthalic acid;hexahydrophthalic acid; tetrachlorophthalic acid; adipic acid; azelaicacid; sebacic acid; succinic acid; maleic acid; glutaric acid; malonicacid; pimelic acid; suberic acid; 2-2-dimethylsuccinic acid;3,3-dimethylglutaric acid; 2,2-dimethylglutaric acid; maleic acid;fumaric acid; and itaconic acid. Anhydrides of the above acids, wherethey exist, can also be employed and are encompassed by the term"polycarboxylic acid." In addition, certain substances which react in amanner similar to acids to form polyesters are also useful. Suchsubstances include lactones such as caprolactone, propylolactone andmethyl caprolactone, and hydroxy acids such as hydroxy caproic acid anddimethylol propionic acid. If a triol or higher hydric alcohol is used,a monocarboxylic acid, such as acetic acid and benzoic acid may be usedin the preparation of the polyester resin. Moreover, polyesters areintended to include polyesters modified with fatty acids or glycerideoils of fatty acids (i.e., conventional alkyd resins). Alkyd resinstypically are produced by reacting the polyhydric alcohols,polycarboxylic acids, and fatty acids derived from drying, semi-drying,and non-drying oils in various proportions in the presence of a catalystsuch as litharge, sulfuric acid, or a sulfonic acid to effectesterification. Examples of suitable fatty acids include saturated andunsaturated acids such as stearic acid, oleic acid, ricinoleic acid,palmitic acid, linoleic acid, linolenic acid, licanic acid, elaeostearicacid, and clupanodonic acid.

Epoxy resins, often referred to simply as "epoxies", are generally knownand refer to compounds or mixtures of compounds containing more than one1,2-epoxy group of the formula ##STR1## i.e., polyepoxides. Thepolyepoxides may be saturated or unsaturated, aliphatic, cycloaliphatic,aromatic or heterocyclic. Examples of suitable polyepoxides include thegenerally known polyglycidyl ethers of polyphenols and/or polyepoxideswhich are acrylic resins containing pendant and/or terminal 1,2-epoxygroups. Polyglycidyl ethers of polyphenols may be prepared, for example,by etherification of a polyphenol with epichlorohydrin or dichlorohydrinin the presence of an alkali. Examples of suitable polyphenols include:1,1-bis(4-hydroxyphenyl)ethane; 2,2-bis(4-hydroxyphenyl)propane;1,1-bis(4-hydroxyphenyl)isobutane;2,2-bis(4-hydroxytertiarybutylphenyl)propane;bis(2-hydroxynaphthyl)methane; 1,5-dihydroxynaphthalene;1,1-bis(4-hydroxy-3-allylphenyl)ethane; and the hydrogenated derivativesthereof. The polyglycidyl ethers of polyphenols of various molecularweights may be produced, for example, by varying the mole ratio ofepichlorohydrin to polyphenol in known manner.

Epoxy resins also include the polyglycidyl ethers of mononuclearpolyhydric phenols such as the polyglycidyl ethers of resorcinol,pyrogallol, hydroquinone, and pyrocatechol.

Epoxy resins also include the polyglycidyl ethers of polyhydric alcoholssuch as the reaction products of epichlorohydrin or dichlorohydrin withaliphatic and cycloaliphatic compounds containing from two to fourhydroxyl groups including, for example, ethylene glycol, diethyleneglycol, triethylene glycol, dipropylene glycol, tripropylene glycol,propane diols, butane diols, pentane diols, glycerol, 1,2,6-hexanetriol,pentaerythritol, and 2,2-bis(4-hydroxycyclohexyl)propane.

Epoxy resins additionally include polyglycidyl esters of polycarboxylicacids such as the generally known polyglycidyl esters of adipic acid,phthalic acid, and the like.

Addition polymerized resins containing epoxy groups may also beemployed. These polyepoxides may be produced by the additionpolymerization of epoxy functional monomers such as glycidyl acrylate,glycidyl methacrylate and allyl glycidyl ether optionally in combinationwith ethylenically unsaturated monomers such as styrene, alpha-methylstyrene, alpha-ethyl styrene, vinyl toluene, t-butyl styrene,acrylamide, methacrylamide, acrylonitrile, methacrylonitrile,ethacrylonitrile, ethyl methacrylate, methyl methacrylate, isopropylmethacrylate, isobutyl methacrylate, and isobornyl methacrylate.

Many additional examples of epoxy resins are described in the Handbookof Epoxy Resins, Henry Lee and Kris Neville, 1967, McGraw Hill BookCompany.

When desired, generally known crosslinking agents may be utilized in themethod of the invention particu[arly when thermosetting resinscontaining active hydrogen atoms are employed in the coatingcompositions.

As will be appreciated by one skilled in the art, the choice ofcrosslinking agent depends on various factors such as compatibility withthe film-forming resin, the particular type of functional groups on thefilm-forming resin and the like. The crosslinking agent may be used tocrosslink the film-forming resin either by condensation or addition orboth. When the thermosetting reactants include monomers havingcomplementary groups capable of entering into crosslinking reactions,the crosslinking agent may be omitted if desired.

Representative examples of crosslinking agents include blocked and/orunblocked diisocyanates, diepoxides, aminoplasts and phenoplasts. Whenaminoplast resins are employed as crosslinking agents, particularlysuitable are the melamine-formaldehyde condensates in which asubstantial proportion of the methylol groups have been etherified byreaction with a monohydric alcohol such as those set forth previously inthe description of aminoplast resins suitable for use as film-formingresins in compositions of the invention.

The term "solvent system" as used herein, for example in the phrase"solvent system for the film-forming resin", is employed in a broadsense and is intended to include true solvents as well as liquiddiluents for the film-forming resin which are not true solvents for thefilm-forming resin. The solvent system may be organic or aqueous. It maybe a single compound of a mixture of compounds. When the solvent systemcomprises both water and an organic portion, the components are usuallymiscible in the proportions employed. The relationship between thesolvent system and the film-forming resin depends upon the absolute andrelative natures of these materials and upon the relative amounts used.Such factors as solubility, miscibility, polarity, hydrophilicity,hydrophobicity, lyophilicity and lyophobicity are some of the factorswhich may be considered. Illustrative of suitable components of thesolvent system which may be employed are alcohols such as lower alkanolscontaining 1 to 8 carbon atoms including methanol, ethanol, propanol,isopropanol, butanol, sec-butyl alcohol, tertbutyl alcohol, amylalcohol, hexyl alcohol and 2-ethylhexyl alcohol; ethers and etheralcohols such as ethylene glycol monoethyl ether, ethylene glycolmonobutyl ether, ethylene glycol dibutyl ether, propylene glycolmonomethyl ether, diethylene glycol monobutyl ether, diethylene glycoldibutyl ether, dipropylene glycol monoethyl ether, and dipropyleneglycol monobutyl ether; ketones such as methyl ethyl ketone, methylisobutyl ketone, methyl amyl ketone and methyl N-butyl ketone; esterssuch as butyl acetate, 2-ethoxyethyl acetate and 2-ethylhexyl acetate;aliphatic and alicyclic hydrocarbons such as the various petroleumnaphthas and cyclohexane; aromatic hydrocarbons such as toluene andxylene; and water.

The basecoating composition also contains a pigment. Examples ofopacifying pigments include titanium dioxide (rutile or anatase), zincoxide, zirconium oxide, zinc sulfide, and lithopone. Examples ofcoloring pigments include iron oxides, cadmium sulfide, carbon black,phthalocyanine blue, phthalocyanine green, indanthrone blue, ultramarineblue, chromium oxide, burnt umber, benzidine yellow and toluidine red.Examples of reactive pigments include silicate-treated bariummetaborate, strontium chromate and lead chromate. Examples of extenderpigments include pigmentary silica, barytes, calcium carbonate, bariumsulfate, talc, aluminum silicates, sodium aluminum silicates, potassiumaluminum silicates and magnesium silicates. Metallic pigments includemetallic powders and metallic flakes. Examples of metallic powdersinclude aluminum powder, copper powder, bronze powder and zinc dust.Examples of metallic flakes include aluminum flakes, nickel flakes,copper flakes, bronze flakes, brass flakes and chromium flakes. A singlepigment may be used or mixtures of pigments may be employed. It ispreferred that at least a portion of the pigment particles be metallicflakes. The metallic flakes usually comprise aluminum flakes.

The principles respecting the formation of solutions, dispersions,pseudodispersions, and emulsions of film-forming resins are generallyknown in the art. Any of these systems may be utilized in thebasecoating and/or topcoating composition.

In addition to the above components, the basecoating and/or thetopcoating compositions employed in the invention may contain optionalingredients which may be employed in their customary amounts for theircustomary purposes provided they do not seriously interfere with goodcoatings practice. Examples of these optional ingredients includevarious fillers; plasticizers; antioxidants; mildewcides and fungicides;surfactants; various catalysts to promote drying or curing; resinouspigment dispersants or grinding vehicles; various flow control agentsincluding, for example, thixotropes and additives for sag resistanceand/or pigment orientation based on organic polymer microparticles(sometimes referred to as microgels) described for example in U.S. Pat.Nos. 4,025,474; 4,055,607; 4,075,141; 4,115,472; 4,147,688; 4,180,489;4,242,384; 4,268,547; 4,220,679; and 4,290,932 the disclosures of whichare hereby incorporated by reference; and other such formulatingadditives. In one embodiment of the present invention organic polymericmicroparticles which are insoluble in the solvent system of thebasecoating composition and which have a diameter in the range of fromabout 0.01 to about 40 microns are utilized in the basecoatingcomposition while substantially inorganic microparticles ranging in sizefrom about 1 nanometer (nm) to about 150 nm, preferably from about 1 nmto about 100 nm, and most preferably from about 3.5 nm to about 50 nm,and preferably comprising silica, are stably dispersed in the topcoatingcomposition. In this embodiment, for example, where an organosol ofsilica particles in an alcohol such as dipropylene glycol monomethylether (as described infra) is incorporated in the topcoatingcomposition, not only is sagging of the topcoating compositionalleviated during heat curing, but the resulting cured topcoat hasexcellent clarity (i.e., transparency).

In a preferred embodiment of the method of the invention the basecoatingcomposition contains substantially inorganic microparticles dispersed inthe basecoating composition. These inorganic microparticles prior toincorporation in the basecoating composition range in diameter fromabout 1 to about 150 nanometers (i.e., from about 1 to about 150millimicrons) preferably from about 1 to about 100 nanometers, and mostpreferably from about 3.5 to about 50 nanometers. A particularlyeffective type of substantially inorganic microparticles for theinvention includes various silicas (especially silica sols) of particlesize within the aforesaid range.

The microparticles suitable for the process of the present invention aresubstantially inorganic. They may or may not have been surface-treatedor surface-modified with various agents to modify the compatibility ofthe microparticles with the film-forming resin or various solventsystems such as polar organic solvents, nonpolar organic solvents, watermiscible organic solvents, water immiscible organic solvents, water andvarious mixtures of two or more of the above. The substantiallyinorganic microparticles can, for example, comprise a core ofessentially a single inorganic oxide such as silica or alumina,preferably silica, or an inorganic oxide of one type coated with aninorganic oxide of another type. However the microparticles suitable forthe present color plus clear coating system, particularly where utilizedin the transparent topcoat, ordinarily are essentially colorless so asnot to seriously interfere with the light transmissive characteristicsof the coating composition when unpigmented. The substantially inorganicmicroparticles may be prepared in any manner capable of providing thedesired particle dimensions suitable for the method of the presentinvention. It is to be understood that although the substantiallyinorganic microparticles may be discrete or associated through physicaland/or chemical means into aggregates, although discrete particles arepreferred, and although a given sample of the microparticles generallywill have particles falling into a range of particle sizes, thesubstantially inorganic microparticles will have an average diameter inthe range of from about 1 to about 150 nanometers. The substantiallyinorganic microparticles used as starting material for incorporation inthe basecoating composition and optionally topcoating composition shouldbe in a form suitable for dispersion in the coating composition wherebyafter dispersing, the substantially inorganic microparticles remainstably dispersed for a period of time at least sufficient so as not toprevent the use of the coating composition in the method of theinvention for its intended purpose. For example, a coating compositioncontaining dispersed inorganic microparticles, depending on the size ofthe inorganic microparticles and the nature of the other componentsemployed in preparing the coating composition, in which the dispersedinorganic microparticles tend to settle over a period of time, but whichcan be redispersed as for example utilizing conventional paint mixingtechniques, is a suitable composition for the color plus clear coatingmethod of the invention.

A particularly desirable class of substantially inorganic microparticlesfor the present invention includes a wide variety of small-particle,amorphous silicas having an average particle diameter ranging from about1 to about 150 nanometers (nm), preferably from about 1 to about 100 nm,and most preferably from about 3.5 to about 50 nm. Such silicas can beprepared by a variety of techniques in a variety of forms examples ofwhich include but are not limited to aquasols, organosols, mixed solsand powder form. As used herein the term "mixed sols" is intended toinclude those dispersions of amorphous silica in which the dispersingmedium comprises both an organic liquid and water. Such small particleamorphous silicas are readily available, are essentially colorless, andhave refractive indices which make them suitable for combination with avariety of film-forming resins and solvent systems so as to formsubstantially colorless transparent coating compositions when thecoating compositions are free of dyes and pigments. Moreover silicas ofappropriate particle size and which have various degrees ofhydrophobicity, hydrophilicity, organophobicity and organophilicity maybe employed depending on compatibility with the particular film-formingresin and solvent system utilized in the method of the invention and onother factors such as the desired degree of room temperature and/or "hotroom" stability of a coating composition comprising silica assubstantially inorganic microparticles.

The silicas ordinarily used in the method of the invention includecommon amorphous forms having ultimate particles of silica which atleast prior to incorporation in the basecoating and/or topcoatingcomposition are essentially unaggregated, the surface of which silicascan contain for example anhydrous SiO₂ groups, SiOH groups, adsorbedorganic groups, chemically bonded organic groups, various ionic groupsphysically associated or chemically bonded to the surface of the silica,and combinations of the above depending on the particularcharacteristics of the silica desired. They can be in the form ofgenerally known organosols, mixed sols, hydrosols, pyrogenic or fumedsilicas, and the like. However where high-solids, organic solvent basedcoating compositions are employed in the method of the invention, it istypically preferred to employ silica organosols of the types in whichthe particles of silica are dispersed in an alcoholic medium such as amonohydric alcohol, a polyol, or a mixture thereof and in which at leasta portion of the surface of the silica particles have been modifiedthrough association with the monohydric alcohol or polyol throughphysical means, chemical bonding or a combination of both.

For example in one preferred embodiment of the invention the inorganicmicroparticles consist of silica in the form of a colloidal dispersionof the silica in an alcohol such as a lower monohydric alcohol examPlesof which include methanol, ethanol, n-propanol, isopropanol, n-butanol,and ether-containing alcohols such as ethylene glycol monomethyl ether,ethylene glycol monoethyl ether, diethylene glycol monomethyl ether,propylene glycol monomethyl ether, dipropylene glycol monomethyl ether,dipropylene glycol monoethyl ether, propylene glycol monobutyl ether,and dipropylene glycol monobutyl ether. Techniques for preparing suchdispersions of colloidal silica in alcohols, as can be seen infra, areknown. Such dispersions are often prepared by controlled addition of anaqueous silica sol to the alcohol while simultaneously removing water,for example by distillation at a temperature below that at whichsubstantial chemical reaction between the hydroxyl groups of the alcoholand silanol groups of the colloidal silica occurs. The products aresometimes referred to as alcosols.

In another preferred embodiment, the inorganic microparticles consist ofsilica in the form of a colloidal dispersion of the silica in an alcoholwherein the surfaces of the silica particles have been made to havevarious degrees of hydrophobicity and organophilicity through chemicalreaction of surface silanol moieties of the silica with an organiccompound having a functional group reactive with the silanol moiety suchthat after reaction, the surface of the silica particles are more orless hydrophobic and the silica particles are compatible to variousdegrees with various organic solvents of the types commonly employed forsolvent systems of organic coating compositions. For example, as furtherdisclosed and illustrated infra, examples of the organic compoundinclude monohydric alcohols such as ethyl alcohol, n-propyl alcohol,n-butyl alcohol, n-pentyl alcohol, n-hexyl alcohol, n-heptyl alcohol,n-octyl alcohol, n-nonyl alcohol, n-decyl alcohol, n-undecyl alcohol,lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol,isobutyl alcohol, isoamyl alcohol, 2,2,4-triaethyl hexane-1-ol,isopropyl alcohol, sec-butyl alcohol, sec-amyl alcohol, sec-n-octylalcohol, methyl isobutyl carbinol, 2,4-dimethyl pentane-3-ol,cyclopentanol, cycloheptanol, methanol, allyl alcohol, crotyl alcohol,2-propyn-1-ol, benzyl alcohol, 2-phenyl-ethanol, 3-phenyl-1 -propanol,and alpha-methylbenzyl alcohol. Saturated primary and secondary alcoholsare preferred alcoholic reactants. Preparation of the chemical reactionproducts of such alcohols as those described above with the silanolmoieties of the silica particles typically involves heating the silicain the presence of the alcohol to a temperature sufficient to effectesterification of the surface silanol moieties by the alcohol. Theesterification temperature will depend on the relative reactivities ofthe hydroxyl group of the particular alcohol with the silanol moiety ofthe silica particle. However, ordinarily it is preferred to carry outthe esterification reaction at a temperature of at least 180° C.Depending upon the particular alcohol used, compatibility with varioussolvent systems can be obtained. For example with long branch chainalcohols such as decyl alcohol the products are compatible withhydrocarbon solvents. Or, for example, when an aromatic substitutedalcohol is used for the esterification reaction, such as benzyl alcohol,the products are compatible with aromatic hydrocarbon solvents such asbenzene, toluene and xylene.

In another embodiment of the invention the substantially inorganicmicroparticles comprise silica prepared from the hydrolysis of acompound selected from a tetraalkylorthosilicate, an alkoxypolysiloxane,an alkylalkoxysilane or a mixture thereof in the presence of a base in awater-alcohol medium of pH greater than 7.0. Typically,tetraalkylorthosilicates selected from tetramethylorthosilicate,tetraethylorthosilicate, tetrapropylorthosilicate or a mixture thereofhave been employed to prepare the silica microparticles. However,alkoxypolysiloxanes such as hexaethoxy disiloxane, octaethoxytrisiloxane, and SILBOND-40, a hydrolyzed and condensedtetraethoxysilane available from Stauffer Chemical Company, may beemployed to prepare silica microparticles of suitable particle size.Examples of alkylalkoxysilanes which can be utilized to prepare thesilica microparticles include hydrolyzable alkylalkoxysilanes such asoctyltriethoxysilane, hexyltriethoxysilane, propyltriethoxysilane,decyltriethoxysilane, methyltriethoxysilane, dimethyldimethoxysilane,mixtures thereof, and the like. For example silica microparticles havingvarious degrees of hydrophobicity and organophilicity can be preparedfrom hydrolyzing mixtures of octyltriethoxysilane andtetraethylorthosilicate in basic media wherein the amount of octyltriethoxysilane may vary from about 1 percent to about 20 percent byweight based on the sum of the weights of the octyltriethoxysilane andtetraethylorthosilicate. The compatibility of the resulting silicamicroparticles with solvents such as hexane and methyl amyl ketone tendsto increase as the percent of octyltriethoxysilane in the reactionmixture increases.

The following description is intended to be additionally illustrative ofsome of the types of silica which can serve as inorganic microparticleshaving an average particle diameter ranging from about 1 to about 150nm, preferably from about 1 to about 100 nm, and most preferably fromabout 3.5 to about 50 nm, for the preparation of compositions useful inthe method of the invention. The preparation and properties of thesesilicas are generally known. The silica particles generally may have asurface area ranging from about 20 to about 3000 square meters per gram(m² /g), preferably from about 30 to about 3000 m² /g, and mostpreferably from about 60 to about 850 m² /g and prior to incorporationinto the coating composition may be in the form of dense, discreteultimate particles or aggregates of associated particles, althoughsilicas having discrete ultimate particles are preferred. In eithercase, the average diameter of the particles of silica (taken asapproximate spheres) will fall within the ranges previously set forth.

One common method for making silicas having an average particle sizeranging from about 1 to about 150 nm employs preparation of the silicain an aqueous medium to produce a hydrosol of silica. Silica hydrosolsmay be prepared for example by partially neutralizing an aqueoussolution of an alkali metal silicate, ordinarily sodium silicate, withacid to a pH typically of about 8 to about 9 such that the resultingsodium content of the solution ordinarily is less than about 1 percentby weight based on sodium oxide. A somewhat different, less commonapproach involves reacting the aqueous sodium silicate solution withsufficient acid to make an acidic sol and precipitating the sodium saltin a strongly acidic medium. This alternate approach makes use of theproperty that polysilic acid is temporarily stable at about a pH of 2,and if the sodium salt of the acid used for neutralizing the sodiumsilicate is sufficiently insoluble, it can be precipitated andseparated. Once the salt is separated from the acidic sol, the sol canbe alkalinized to grow colloidal particles and stabilize the product orcan be employed in other known processes of growing silica particles tothe desired size. Typically a heating step is involved in theseprocesses since temperature is a well known variable for controlling theparticle size of the silica product. Representative preparations ofsilica aquasols are contained in the following literature which ishereby incorporated by reference: U.S. Pat. Nos. 2,244,325; 2,375,738;2,574,902; 2,577,484; 2,577,485; 2,750,345; 3,012,973; 3,440,174;3,538,015; 3,673,104; 3,714,064 and THE CHEMISTRY OF SILICA by Ralph K.Iler, copyright 1979, pages 172-176 and 331-343. Aquasols of silica maybe utilized in the method of the invention for example where water-basedcoating compositions are employed. They also can be used for example asstarting materials for the preparation of organosols.

A method for preparing silica sols having uniform spherical silicaparticles of controlled particle size by hydrolyzing a loweralkoxysilane in an alcohol medium containing suitable amounts of waterand ammonia has been described by Stober et al in the JOURNAL OF COLLOIDAND INTERFACE SCIENCE, Volume 26, pages 62-69 (1968) the disclosure ofwhich is hereby incorporated by reference.

Silica organosols are especially preferred in the method of the presentinvention where organic solvent based coating compositions are employed.The silanol (SiOH) surface of silica particles which have not beenmodified utilizing various organic compounds tends to limit theirdispersibility in organic media to organic liquids such as loweralcohols, amides and ketones. However silicas containing surfacemodification by various organic compounds can be dispersed to formorganosols in a variety of organic liquids. Such surface modificationmay be in the form of treatment of the silica with organic ions such asquaternary ammonium ions like tetramethylammonium ions, quaternaryammonium ions containing long-chain hydrocarbon groups and by treatingwith an organic amine or quaternary ammonium compound in the presence ofa water-immiscible organic liquid as described for example in U.S. Pat.Nos. 2,601,352; 2,692,863 and 3,629,139 and THE CHEMISTRY OF SILICA byRalph K. Iler, copyrignt 1979, pages 359 and 360. Organic anions can beattached to the silica surface through metal cations as described forexample in U.S. Pat. No. 3,625,856. Surface modification of the silicaparticles can be effected by treatment with various organic compoundswhich can associate with the surface of the silica either throughphysical means and/or chemical means as for example by treatment withmonohydric alcohols, polyols, and mixtures thereof, optionally underconditions of time and temperature such that silanol groups of thesilica particles are esterified by reaction with hydroxyl groups of themonohydric alcohols and/or polyols. Still another way to modify thesurface of a silica particle to render it more organophilic is to reactthe surface of the silica with alkyl chlorosilanes to attach organosilylgroups. In short, a variety of ways are well known in the art formodifying the surface of silica to make the silica more compatible withorganic liquids. Representative preparations of silica organosols andmixed sols can be found in the following literature which is herebyincorporated by reference: U.S. Pat. Nos. 2,375,738; 2,577,485;2,801,185; 2,801,186; 3,108,970; 2,692,863; 3,629,139; 3,660,301;2,601,352; 3,625,856; 2,921,913; 2,739,078; 3,699,049; 2,974,105;3,351,561; 3,336,235; 3,855,145; 2,786,042; and THE CHEMISTRY OF SILICAby Ralph K. Iler, copyright 1979, pages 412-415.

Silicas in powder form represent another, although less preferred, typeof silica which may be utilized as the substantially inorganicmicroparticles in the method of the invention. Silica powders may beprepared for example from crushing a silica gel or from generally knownflame hydrolysis techniques which result in what are often referred toas fumed silicas or pyrogenic silicas. Silica powders generally are in aform in which the particles consist of small granules of pulverizedsilica gel or of coherent aggregates of submicron particles that arelinked together in networks. As disclosed by R. K. Iler in THE CHEMISTRYOF SILICA, copyright 1979 at page 462, 476 and 477, a silica powdermight consist of separate discrete silica particles, but when theparticle diameter is less than 100 nm, the particles spontaneouslyadhere into loose aggregates. In powders the ultimate (or primary)particles are aggregated into what have been called "secondaryparticles", "clusters" or "aggregate particles" all of which arereferred to herein simply as aggregates. Various types of silica powdersare known in which the nature of the surfaces of the silica particlesvary from one type of powder to another. For example, in some powdersthe surface of the particles are essentially fully hydroxylated, i.e.,the surface structure terminates in silanol (SiOH) groups. Powders ofthe fully hydroxylated type tend to be readily wettable by water andwater-miscible organic molecules. In some powders, the surface of theparticles can be considered to be made up of siloxane ##STR2## bondswhich present primarily oxygen atoms at the surface. In these powders,usually only a small fraction of silanol groups are present. Pyrogenicsilicas condensed from the vapor state as well as hydroxylated silicaswhich have been dehydrated at high temperatures, e.g., about 1000° C.,tend to be of this type. Moreover, the surfaces of silica powders can bemodified by chemical or physical attachment of organic molecules orradicals which modification can impart various degrees ofhydrophobicity, organophilicity, hydrophilicity, or oleophobicity to thesilica particles.

The silicas in powder form, for reasons which are not entirelyunderstood, tend to perform somewhat less effectively in the method ofthe invention. When employed in the basecoating composition for examplethey typically require a disadvantageous amount of grinding whichtypically is done as pregrinding in a solvent. Moreover, when employedin the topcoating composition, the dried or cured film often does notexhibit as high a degree of gloss as is desired for certain applicationssuch as topcoats for automobiles. The differences in behavior of thepowder forms of silica compared to for example silicas in the form oforganosols, is believed to be at least in part a result of a highproportion of particles in the form of aggregates being present in thepowder forms. Moreover, when utilized in a basecoating cospositioncontaining metallic pigment, typically a larger amount of a powder formof silica is needed to provide a desired degree of metallic patterncontrol as compared to utilization of a silica in the form of a solhaving a high proportion of discrete ultimate particles.

A wide variety of silicas in the form of hydrosols, organosols andpowders may be obtained for example under the trade names of LUDOX fromE. I. Du Pont De Nemours and Company, NALCOAG from Nalco ChemicalCompany, NYACOL from Nyacol, Inc., SNOWTEX from Nissan ChemicalIndustries, Ltd., CAB-O-SIL from Cabot Corp. and AEROSIL from DegussaInc.

The basecoating composition and topcoating compositions are usuallyprepared by simply admixing the various ingredients for the respectivecompositions at room temperature although elevated temperatures may beused.

The amounts of the materials in the basecoating composition includingthe substantially inorganic microparticles can vary widely. Generallythe film-forming resin constitutes from about 10 percent to about 95percent by weight, typically from about 25 percent to about 50 percentby weight, of the basecoating composition. Generally the amount ofsubstantially inorganic microparticles can range from about 1 percent toabout 30 percent by weight, typically from about 10 percent to about 20percent by weight, based on the sum of the weights of the organicfilm-forming resin, optional crosslinking agent, and inorganicmicroparticles.

The amount of solvents and/or diluents constituting the solvent systemfor the film-forming resin also may vary widely. Generally the totalamount of solvents and/or diluents may range from about 0 to about 80percent by weight, typically from about 35 to about 65 percent byweight, of the basecoating composition.

The amounts of pigment particles present in the basecoating compositionis likewise subject to wide variation. Generally the pigment is presentin an amount ranging from about 2 to about 50 percent by weight,typically from about 3 to about 30 percent by weight, based on the sumof the weights of the film-forming resin and the substantially inorganicmicroparticles. When metallic flakes are employed as pigment on thebasecoating composition, they generally are present in the range of fromabout 2 to about 30 percent by weight, typically from about 10 to about20 percent by weight, based on the sum of tha weights of thefilm-forming resin and the substantially inorganic microparticlespresent in the basecoating composition.

The film-forming resin of the topcoating composition can be any of thefilm-forming resins useful for coating compositions and can be the sameor different from the film-forming resin of the basecoating composition.Likewise, film-forming resins for the topcoating composition can befilm-forming thermoplastic resins and/or thermosetting resins.Illustrative examples of film-forming resins suitable for the topcoatingcomposition have been described previously in the discussion of examplesof film-forming resins suitable for the basecoating composition. Thesolvent systems described with respect to the basecoating compositionalso can be employed for the film-forming resin of the topcoatingcomposition. For example, the film-forming resin of the topcoatingcomposition may be dissolved in the solvent system or it may bedispersed in the solvent system. Like the solvent system for thefilm-forming resin of the basecoating composition, the solvent systemfor the topcoating composition may be organic or aqueous and may be asingle compound or a mixture of compounds. Illustrative of componentssuitable for the solvent system include those described previously.

As for the film-forming resin of the basecoating composition thefilm-forming resin of the topcoating composition may be present in thecoating composition in the form of a solution, dispersion, emulsion orpseudodispersion. Likewise, the topcoating composition may containoptional ingredients such as various fillers, plasticizers,antioxidants, mildewcides and fungicides, surfactants, various catalyststo promote drying or curing, and various flow control agents asdescribed previously with respect to the basecoating composition.

Where a crosslinkable film-forming resin is utilized in the topcoatingcomposition, optionally a crosslinking agent can be incorporated in thetopcoating composition. Examples of such crosslinking agents includethose described previously with respect to the basecoating composition.

The topcoating composition is formulated so that when it is applied tothe basecoat, it forms a clear topcoat so that the pigmentation of thebasecoat will be visible through the topcoat. It should be understoodthat the topcoat, while being transparent, may contain small amounts ofdyes and/or tints to modify the overall appearance where desired.However, it is usually preferable not to employ even small amounts ofdyes and/or tints in the topcoating composition. Although the topcoatingcomposition may contain transparent extender pigments and optionally asmall amount of coloring pigment, it should not contain so much coloringpigment that it interferes with the general transparency of the topcoat.Usually it is preferable not to utilize even small amounts of coloringpigment in the topcoating composition.

The amounts of the film-forming resin, solvent system, and optionallyinorganic microparticles, employed in the topcoating compositiongenerally are as described with respect to the amounts of thesecomponents for the basecoating composition. However, when inorganicmicroparticles are incorporated in the topcoating composition forexample to control sagging, the level of inorganic microparticles soutilized is typically less than the level of inorganic microparticleswhen used in the basecoating composition to control pigment orientation.Generally the amount of substantially inorganic microparticles in thetopcoating composition can range from about 1 percent to about 20percent by weight, typically from about 2 percent to about 12 percent byweight, based on the sum of the weights of the organic film-formingresin, optional crosslinking agent, and inorganic microparticles.

The method of the invention can be employed utilizing a wide variety ofsubstrates such as metals, wood, glass, cloth, plastics, fiberglass,foams and the like as well as over primers. The basecoating compositionand topcoating composition can be applied to the substrate using anyapplication technique known in the art such as roll coating, curtaincoating, dip coating, doctor blade coating, spraying and the likealthough spraying is most often employed.

In the method of the invention the basecoating composition containingorganic film-forming resin, the solvent system for the film-formingresin, pigment particles, and dispersed inorganic microparticles, isfirst applied to the substrate. The basecoating composition, dependingon the choice of thermoplastic and/or thermosetting resin, may be driedor cured at ambient temperature or with applied heat to a degree atleast sufficient to allow the clear topcoating composition to be appliedto the basecoat without undesirable strike-in. Thermoplastic coatingcompositions are typically hardened by evaporation of the volatilesolvent system. Thermosetting coating compositions can be cured in avariety of ways, typically at temperatures in the range of from about20° C. to about 260° C. Some of the thermosetting film-forming resinssuch as air-curable alkyds for example may be cured by exposure to theoxygen in air. Many of the coating compositions contain a crosslinkingagent. When a crosslinking agent is present, the coating compositionsare usually cured by the application of heat. Although a curingtemperature may vary widely it is typically in the range of about 80°Celsius (C) to about 150° C. Similarly, curing times may be subject towide variation, but typically range from about 10 minutes to about 45minutes. Where a plurality of superimposed basecoats or topcoats are tobe applied, each coating composition may be cured prior to applicationof the next coating composition. It is preferable, however, to utilizecoating systems which will permit the application of two or moresuperimposed coatings which can be cured together in a single curingoperation. For example, a thermosetting basecoat may be cured prior toapplication of a thermosetting topcoat, although it is preferred to usecoating systems which will permit the topcoating composition to beapplied to a substantially uncured basecoat and to cure themsimultaneously in one operation. Particularly when heat curing isemployed, it is sometimes desirable to allow the basecoating compositionto flash at ambient temperature for up to about 30 minutes, typically upto about 5 minutes, before the topcoating composition is applied to thebasecoat. Such solvent flashing may be utilized with either basecoatingcompositions containing thermoplastic film-forming resins or withbasecoating compositions containing thermosetting film-forming resins(i.e., those which involve some degree of crosslinking during cure).

The color plus clear method of the invention provides a number ofadvantages. By incorporating the inorganic microparticles in thepigmented basecoating composition and optionally in the topcoatingcomposition, the amount of sagging of the coating compositions on averticle substrate during heating can be substantially reduced or eveneliminated often without the use of known organic microgels. Moreover,this advantage with respect to sag control is especially important whena high-solids coating composition is utilized in the method of theinvention where sag control can be an especially serious problem. Asused herein, the term "high solids coating composition" is intended toinclude those coating compositions having a total solids content of atleast about 50 percent by weight, preferably at least about 60 percentby weight, based on the total weight of the coating composition andwhich can be applied to the substrate by conventional sprayingtechniques. Moreover, it is preferred that the basecoating andtopcoating compositions be applied by conventional spraying to thesubstrate at a combined total solids content of at least 50 percent byweight of the sum of the basecoating composition and the topcoatingcomposition. The solids are understood to include the essentiallynonvolatile components of the coating composition including, forexample, film-forming resin, inorganic microparticles and pigmentparticles. It is to be understood that the optional crosslinking agents,examples of which have been described above, are intended to be includedfor the purpose of the determination of the solids content of thecoating composition. Particularly where a high-solids coatingcomposition is utilized in the method of the invention, typically theorganic film-forming resin will comprise a crosslinkable resin having aweight average molecular weight of from about 500 to about 10,000 andtypically the coating composition will contain a crosslinking agentexamples of which include those described previously.

Additionally, when the inorganic microparticles are incorporated in thetopcoating composition, the topcoating composition surprisingly can becured to a high gloss film without the occurrence of substantialflattening effects (i.e., substantial gloss reduction) which certainparticulate silicas which have previously been used in coatingcompositions provide. This is important for example where high glosscoatings are desired as in automotive coatings applications. Equallyimportant, where metallic flakes are employed as pigment in thebasecoating composition, the incorporation of the inorganicmicroparticles provides excellent control of the pigment orientation inthe basecoat such that the dried or cured coating exhibits a high degreeof pattern control as evidenced by excellent two tone appearance whenviewed at different angles to a direction normal to the coated surfaceand excellent metallic brightness (sometimes referred to as brightnessof face or lightness of face) when viewed from a direction essentiallynormal to the coated substrate. Moreover, this high degree of patterncontrol may be achieved in the method of the invention without thenecessity of using known organic polymer microgels which have beensynthesized for this purpose, although it is to be understood that thescope of the present invention is intended to include the color plusclear coating method of the invention wherein an organic polymermicrogel is employed in combination with the inorganic microparticles inthe basecoating composition and/or topcoating composition.

Some further advantages of the method of the invention may obtainbecause of the nature of the inorganic microparticles. Beingsubstantially inorganic, the microparticles are inherently moreresistant to degradation from the action of ultraviolet light as fromexposure to sunlight, from hydrolysis, and from extreme conditions suchas high temperatures and salt spray. Moreover, the inorganicmicroparticles are not subject to internal attack by organic solventsand do not swell in the presence of organic solvents.

The following examples are intended to further illustrate the presentinvention. As used in the body of the specification, examples andclaims, all percents, ratios and parts are by weight unless otherwisespecifically indicated. As used herein, "pbw" means "parts by weight".

EXAMPLE 1

This example illustrates the preparation of a colloidal silica indipropyleneglycol monomethylether.

A 12 liter flask is equipped for vacuum distillation, the apparatusincluding a mechanical stirrer, heating mantle, addition funnel, potthermometer, and vacuum take-off head containing a thermometer and acondenser.

The flask is charged with 5600 g of dipropyleneglycol monomethylether(DOWANOL DPM from Dow Chemical Company).

To the contents of the flask is added 6118 g of aqueous colloidal silica(NALCOAG 1034A, having a silica solids content of 35.3% by weight and apH of about 3). Distillate containing water is removed under a vacuum of45 torr while the temperature of the contents of the flask rises from44° C. to 95° C. during which time essentially all of the water in theflask is distilled off. The resulting dispersion of colloidal silica indipropyleneglycol monomethylether has a content of silica solids of30.8% by weight, a residual water content of 0.27% by weight, and a lowviscosity (12.4 seconds, No. 4 Ford Cup).

EXAMPLE 2

A 50% solids dispersion of silica in dipropyleneglycol monomethyletheris prepared by a process similar to that described in EXAMPLE 1. A 3liter flask is equipped for vacuum distillation as described inEXAMPLE 1. The flask is charged with 1050 g of dipropyleneglycolmonomethylether (DOWANOL DPM from Dow Chemical Company).

To the contents of the flask is added 2578.8 g of aqueous colloidalsilica (NALCOAG 1034A having a silica solids content of 34.9% by weightand a pH of about 3). The addition of the aqueous colloidal silica isdone slowly and the pot temperature is maintained at about 45°-50° C. bythe addition of the aqueous silica. During the addition, distillatecontaining water is removed under a vacuum of 45 torr while thetemperature of the contents of the flask rises from 42° C. to 100° C.during which time essentially all of the water in the flask is distilledoff. The resulting dispersion of colloidal silica in dipropyleneglycolmonomethylether has a content of silica solids of 50.8% by weight, aresidual water content of 0.08% by weight and a viscosity of 18 secondsthrough a No. 4 Ford Cup.

EXAMPLE 3

A dispersion of colloidal silica in dipropyleneglycol monomethyletherwhich is essentially free of water and which has a silica solids contentof 32 percent by weight is prepared according to the procedure describedin EXAMPLE 1.

A 5 liter flask equipped for distillation is charged with 2812.5 gramsof the dispersion of colloidal silica (32 percent by weight silicasolids) and 600 grams of n-decyl alcohol. The contents of the flask areheated and held at reflux under atmospheric pressure for about 2 hoursduring which time the temperature of the contents of the flask graduallyrises to 207° C. and 1931 grams of distillate is removed. The resultingproduct is a colloidal silica having n-decyloxy groups bound to thesilica surface. To the resulting product is added 1900 grams ofmethylamyl ketone to reduce the silica solids content of the flask to26.6 percent by weight.

EXAMPLE 4

A grinding apparatus consisting of a ball mill jar rotated by a pair ofrollers and containing solid, ceramic, Burundum cylinders that range inlength from about 1/2 to about 3/4 inch in length and from about 3/8 toabout 3/4 inch in diameter is charged with 60 grams of a fumed silicahaving a surface area of approximately 200 square meters per gram(available as CAB-O-SIL PTG from Cabot Corporation) and 340 grams ofpropyleneglycol monomethylether (DOWANOL PM). The fumed silica is groundin the propyleneglycol monomethylether for 64 hours. The resultingcomposition has a silica solids content of 16.0 percent by weight (asdetermined at 250° F. for 2 hours) and gives a Hegman grind reading of8+.

EXAMPLE 5

An amount of 225 grams of fumed silica having a surface area of about120 m² /g (available as AEROSIL R972 from Degussa, Inc.) is ground for64 hours in 1275 grams of propyleneglycol monomethylether (DOWANOL PM)according to the procedure described in EXAMPLE 4. The resultingcomposition has a silica solids content of 15.9 percent by weight andgives a Hegman grind reading of 8+.

EXAMPLE 6

A 1 liter flask is equipped for vacuum distillation as described inEXAMPLE 1 and is charged with 460 g of dipropyleneglycol monomethylether(DOWANOL DPM).

An aqueous dispersion of colloidal silica having a silica solids contentof 50 percent by weight and an average silica particle size of greaterthan 700 Angstroms (NALCOAG 5SJ-626) is treated with a strong acid ionexchange resin (Amberlite 200 from Rohm and Haas Company) until the pHis lowered to 2.5.

The aqueous dispersion of colloidal silica having a pH of 2.5 is addedto the 1 liter flask containing the dipropyleneglycol monomethyletherwhile water is distilled off under a vacuum of 40 torr. The vacuumdistillation is continued until the pot temperature reaches 110° C. andthe vapor temperature (head temperature) reaches 90° C. The total amountof distillate recovered is 574 g. The resulting dispersion of colloidalsilica in dipropyleneglycol monomethylether has a silica solids contentof 44.3 percent by weight.

EXAMPLE 7

A 1 liter flask is equipped for vacuum distillation as described inEXAMPLE 1.

An aqueous colloidal silica having a silica solids content of 15 percentby weight and an average silica particle size of 40 Angstroms, A°(NALCOAG 1115) is treated with a strong acid ion exchange resin in theacid form (Amberlite 200 from Rohm and Haas Company) until the pH of theaqueous colloidal silica is lowered to 3.0. Next this acidifiedcolloidal silica is treated with a strongly basic ion exchange resinwhich is in the hydroxide form (Amberlite IRA-900 from Rohm and HaasCompany) until the pH is raised to 5. Thereafter, this colloidal silicais further contacted with Amberlite 200 until the pH is lowered to 2.8.These multiple ion exchange steps are similar to those described in U.S.Pat. No. 3,855,145.

Next, 630 g of the ion exchanged, aqueous colloidal silica having a pHof 2.8 is added slowly to the 1 liter flask which contains 630 g ofdipropyleneglycol monomethylether (DOWANOL DPM). The pot temperature ismaintained at 45°-50° C. during the addition of the ion exchanged,aqueous colloidal silica to the dipropyleneglycol monomethylether.Thereafter, water is distilled off under a vacuum of 40 torr while thepot temperature rises to 71° C. and the vapor temperature (headtemperature) rises to 54° C. A total of 586 g of distillate isrecovered. The resulting dispersion of colloidal silica indipropyleneglycol monomethylether has a silica solids content of 12.8percent by weight and a residual water content of 1.08 percent byweight.

EXAMPLE 8

A 5 liter, 4-neck flask is equipped with a stirrer, thermometer,condenser, and distillation take-off.

In the 5 liter flask are mixed 2402 g of methanol, 460 g of distilledwater, and 156.1 g of an aqueous solution containing 30 percent byweight ammonia. The resulting solution is heated to reflux and when theconcentration of ammonia in the solution is 0.65 molar, 532 g ofethylsilicate, condensed, from Union Carbide Corporation (believed tocontain about 90 percent by weight tetraethylorthosilicate and about 10percent by weight of hexaethoxydisiloxane) is added to the contents ofthe flask all at once with vigorous stirring. The contents of the flaskturns milky 4 minutes after the addition of the ethylsilicate,condensed, to the flask.

Next, the contents of the flask are refluxed for two hours andthereafter allowed to cool to room temperature.

Next, 1695 g of a distillate containing methanol and ammonia is removedfrom the flask by vacuum distillation at 90 torr and 32° C.

The dispersion remaining in the flask is then acidified from a pH of8.44 to a pH of 2.40 by the addition of 8 milliliters (ml) ofconcentrated hydrochloric acid (12 Molar). To the acidified colloidaldispersion is added 1200 ml of 1-methoxy-2-propanol (DOWANOL PM from DOWChemical Company).

Next, essentially all of the remaining methanol and water in addition tosome of the 1-methoxy-2-propanol are removed by azeotropic distillationat a pressure of 45 torr. Thereafter, further concentration of thecolloidal dispersion by vacuum distillation at 45 torr and 62° C.results in a stable dispersion of colloidal silica in1-methoxy2-propanol having a silica solids content of 24.2 percent byweight. During the aforesaid azeotropic distillation and furtherconcentration at 45 mm torr, a total of 1994.5 g of distillate isremoved from the flask.

EXAMPLE 9

A 5 liter, 4-neck flask equipped with a stirrer, thermometer, anddistillation take-off is charged with 2039 grams of methanol, 220 gramsof distilled water, and 216.6 grams of an aqueous solution containing 30percent by weight ammonia. The resulting solution is heated to refluxand when the concentration of ammonia in the solution is 0.71 molar, amixture of 504.7 grams of ethyl, silicate, condensed, from Union CarbideCorporation (believed to contain about 90 percent by weighttetraethylorthosilicate and about 10 percent by weight ofhexaethoxydisiloxane) and 21.0 grams of octyltriethoxysilane is addedall at once with vigorous stirring.

Next, the contents of the flask are refluxed for two hours andthereafter allowed to cool to room temperature.

Next, 1045 grams of a distillate containing methanol and ammonia isremoved from the flask by vacuum distillation at 50 torr pressure and23° C.

Next, 300 milliliters of 1-methoxy-2-propanol (DOWANOL PM from DOWChemical Company) is added to the contents of the flask and thedispersion is thereafter acidified from a pH of 8.9 to a pH of 2.26 bythe addition of 10 ml of concentrated hydrochloric acid (12 Molar). Tothe acidified colloidal dispersion is added 1000 ml of1-methoxy-2-propanol (DOWANOL PM).

Next, essentially all of the remaining methanol and water in addition tosome of the 1-methoxy-2-propanol are removed by azeotropic distillationat a pressure of 45 torr and a temperature of 59° C. During theaforesaid azeotropic distillation a total of 2425 g of distillate isremoved from the flask. The resulting product is a stable dispersion ofcolloidal silica in 1-methoxy-2-propanol having a silica solids contentof 19.7 percent by weight.

EXAMPLE 10

The following Examples 10A through 10I illustrate the method of thepresent invention utilizing coating compositions prepared from thedispersions of colloidal silica of EXAMPLES 1 through 9. Example 10J isa comparison example.

High-solids, metallic basecoating compositions are prepared by mixingunder agitation the ingredients in the amounts in parts by weight setforth in the following TABLE 1.

                                      TABLE 1                                     __________________________________________________________________________    Composition   10A                                                                              10B                                                                              10C                                                                              10D 10E 10F 10G                                                                              10H                                                                              10I                                                                              10J                               __________________________________________________________________________    Ultraviolet light absorber.sup.1                                                            3.0                                                                              -- -- --  --  --  -- -- -- --                                Methylamyl ketone                                                                           11.6                                                                             36.5                                                                             -- --  --  --  -- -- -- 47.5                              Colloidal silica dispersion.sup.2                                                           65.6                                                                             -- -- --  --  --  -- -- -- --                                Colloidal silica dispersion.sup.3                                                           -- 41.2                                                                             -- --  --  --  -- -- -- --                                Colloidal silica dispersion.sup.4                                                           -- -- 78.9                                                                             --  --  --  -- -- -- --                                Colloidal silica dispersion.sup.5                                                           -- -- -- 140.6                                                                             --  --  -- -- -- --                                Colloidal silica dispersion.sup.6                                                           -- -- -- --  188.7                                                                             --  -- -- -- --                                Colloidal silica dispersion.sup.7                                                           -- -- -- --  --  152.4                                                                             -- -- -- --                                Colloidal silica dispersion.sup.8                                                           -- -- -- --  --  --  82.0                                                                             -- -- --                                Colloidal silica dispersion.sup.9                                                           -- -- -- --  --  --  -- 80.6                                                                             -- --                                Colloidal silica dispersion.sup.10                                                          -- -- -- --  --  --  -- -- 98.5                                                                             --                                Acrylic resin.sup.11                                                                        43.26                                                                            70.1                                                                             43.26                                                                            70.06                                                                             60.1                                                                              10.1                                                                              86.1                                                                             74.06                                                                            74.1                                                                             100.0                             Crosslinking agent.sup.12                                                                   75.0                                                                             75.0                                                                             75.0                                                                             75.0                                                                              75.0                                                                              75.0                                                                              75.0                                                                             75.0                                                                             75.0                                                                             75.0                              Methanol      9.0                                                                              9.0                                                                              9.0                                                                              9.0 9.0 9.0 9.0                                                                              9.0                                                                              9.0                                                                              9.0                               Blue pigment paste.sup.13                                                                   86.4                                                                             -- 86.4                                                                             --  --  --  -- -- -- --                                Aluminum pigment paste.sup.14                                                               -- 31.6                                                                             -- 31.6                                                                              31.6                                                                              31.6                                                                              31.6                                                                             31.6                                                                             31.6                                                                             31.6                              Catalyst composition.sup.15                                                                 3.8                                                                              3.8                                                                              3.8                                                                              3.8 3.8 3.8 3.8                                                                              3.8                                                                              3.8                                                                              3.8                               __________________________________________________________________________     .sup.1 A derivative of hydroxyphenyl benzotriazole available as TINUVIN       328 from Geigy Industrial Chemicals.                                          .sup.2 A dispersion of colloidal silica in dipropylene glycol                 monomethylether prepared in the manner described in EXAMPLE 1 but having      silica solids content of 32.0 percent by weight.                              .sup.3 The dispersion of colloidal silica in dipropyleneglycol                monomethylether as described in Example 2.                                    .sup.4 The dispersion of colloidal silica in dipropyleneglycol                monomethylether as described in Example 3.                                    .sup.5 The dispersion of colloidal silica in propyleneglycol                  monomethylether as described in Example 4.                                    .sup.6 The dispersion of colloidal silica in propyleneglycol                  monomethylether as described in Example 5.                                    .sup.7 The dispersion of colloidal silica in dipropyleneglycol                monomethylether as described in Example 6.                                    .sup.8 The dispersion of colloidal silica in dipropyleneglycol                monomethylether as described in Example 7.                                    .sup.9 The dispersion of colloidal silica in 1methoxy-2-propanol as           described in Example 8.                                                       .sup.10 The dispersion of colloidal silica in 1methoxy-2-propanol as          described in Example 9.                                                       .sup.11 A high solids acrylic resin at 75 percent by weight acrylic resin     solids in ethyleneglycol monomethylether acetate available as AT400-CA        from Rohm and Haas Company.                                                   .sup.12 A methylated and butylated melamine crosslinking agent available      as CYMEL 1130 from American Cyanamid Company.                                 .sup.13 A composition prepared from 23.75 pbw of nonleafing aluminum          flakes, 1.25 pbw of Monarch Blue Pigment from Ciba Geigy Corporation, 25      pbw of an iminated acrylic grinding resin, and 50 pbw of solvent              comprising methylamyl ketone, mineral spirits, Naphtholite ® ,            isobutanol and toluene.                                                       .sup.14 A dispersion of nonleafing aluminum flakes at 65 percent by weigh     solids in mineral spirits available as Aluminum Paste R167 from Ohio          Bronze Powder Company.                                                        .sup.15 A mixture of 55 percent by weight of dinonylnaphthalene disulfoni     acid in isobutanol of which 50 mole percent of the acid groups have been      neutralized with diisopropanol amine is combined with sufficient              isopropanol and water to produce the catalyst composition containing 30.0     percent by weight dinonylnaphthalene disulfonic acid, 7.5 percent by          weight diisopropanol amine, and 62.5 percent by weight solvents (52.58        percent isopropanol, 39.27 percent isobutanol, and 8.15 percent water).  

Compositions 10A through 10J are reduced to a No. 4 Ford Cup viscosityof 19-22 seconds with methylamyl ketone to provide basecoatingcompositions 10A through 10J and are spray applied to unprimed steelpanels using conventional spraying equipment to provide a dry filmthickness of basecoat of about 0.6 to 0.7 mils. The spray viscosity andpercent by weight spray solids are as set forth in the following TABLE2. Next, the basecoating compositions on the steel panels are allowed toflash for 2 minutes at ambient atmospheric conditions. Immediatelythereafter, a conventional, clear automotive acrylic topcoatingcomposition (available as DCT-2000 from PPG INDUSTRIES, INC.) is sprayapplied in two passes to the basecoated steel panels with a one minuteflash at ambient atmospheric conditions between passes.

Next, the basecoating and topcoating compositions on the steel panelsare cured at 250°F. (121° C.) for 30 minutes. All of the cured filmsprepared utilizing compositions 10A through 10I exhibited good toexcellent metallic pattern control as determined visually compared tothe cured film prepared utilizing composition 10J which contained nocolloidal silica. Moreover the wet coatings prepared utilizingcompositions 10A through 10I exhibit good resistance to sag during theheat curing step while the wet coating prepared utilizing composition10J exhibits much sagging after application and the heat curing step.

Additionally, the 20° gloss, distinctness of image (DOI), and dry filmthickness (DFT) in mils is determined for each of the cured compositecoatings and the results are as set forth in the following TABLE 3.

                  TABLE 2                                                         ______________________________________                                                    Spray Viscosity                                                                            Percent by Weight                                    Composition (No. 4 Ford Cup)                                                                           Spray Solids                                         ______________________________________                                        10A         19.5         49.3                                                 10B         19.5         51.3                                                 10C         19.3         52.2                                                 10D         21.3         37.6                                                 10E         20.6         43.9                                                 10F         20.6         47.7                                                 10G         19.6         42.9                                                 10H         19.7         52.4                                                 10I         20.8         52.2                                                 10J         20.3         59.1                                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Example  20° Gloss                                                                            DOI    DFT (mils)                                      ______________________________________                                        10A      84-88         66.2   2.15                                            10B      87            47.1   2.25                                            10C      87-88         62.6   2.15                                            10D      94-95         61.2   2.0                                             10E      89            60.5   2.2                                             10F      68            18.5   2.05                                            10G      52            14.5   2.05                                            10H      86-89         53.0   2.3                                             10I      87            43.0   2.1                                             10J      40-45          9.0   1.8-2.4*                                        ______________________________________                                         *Exhibits much sag.                                                      

EXAMPLE 11

This example illustrates the method of the invention utilizing ahigh-solids basecoating composition.

A high-solids, metallic basecoating composition is prepared by mixingunder agitation the ingredients in the amounts in parts by weight setforth in the following TABLE 4.

                  TABLE 4                                                         ______________________________________                                                          pbw                                                         ______________________________________                                        Ultraviolet light absorber.sup.1                                                                   3.0                                                      Methylamyl ketone   20.0                                                      Colloidal silica dispersion.sup.2                                                                 70.3                                                      Acrylic resin.sup.3 70.1                                                      Crosslinking agent.sup.4                                                                          75.0                                                      Methanol             9.0                                                      Aluminum paste.sup.5                                                                              31.6                                                      Catalyst composition.sup.6                                                                         3.8                                                      ______________________________________                                         .sup.1 As described in footnote 1 to Table 1.                                 .sup.2 A dispersion of colloidal silica in dipropyleneglycol                  monomethylether prepared in the manner described in Example 1 but having      silica solids content of 32.0 percent by weight.                              .sup.3 As described in footnote 11 to Table 1.                                .sup.4 As described in footnote 12 to Table 1.                                .sup.5 As described in footnote 14 to Table 1.                                .sup.6 As described in footnote 15 to Table 1.                           

The metallic basecoating composition is spray applied at a solidscontent of 57.4 percent by weight and a No. 4 Ford Cup viscosity of 39.6seconds to an unprimed steel panel to form a basecoat. Next, thebasecoat is allowed to flash for 2 minutes at ambient atmosphericconditions. Immediately thereafter, a conventional, clear, automotiveacrylic topcoating composition (available as DCT-2000 from PPGINDUSTRIES, INC.) is spray applied in two passes to the basecoat with aone minute flash at ambient atmospheric conditions between passes toform a topcoat.

Next the basecoat and topcoat are cured at 250° F. (121° C.) for 30minutes to a total dry film thickness of 2.1 mils. The composite coatingexhibits excellent resistance to sagging during cure and the resultingcured composite film exhibits excellent metallic pattern control asdetermined visually (i.e., the cured film exhibits an excellent two toneappearance when viewed at different angles to a direction normal to thesurface and excellent metallic brightness when viewed from a directionessentially normal to the surface). Additionally the cured compositefilm has a 20° gloss of 86 and a definition of image (DOI) of 52.7 atthe 2.1 mil dry film thickness.

EXAMPLE 12

This example illustrates the method of the invention wherein thetopcoating composition containing colloidal silica is applied to abasecoat which does not contain colloidal silica.

Topcoating composition 12A and topcoating composition 12B (forcomparison) are prepared by mixing under agitation the ingredients inthe amounts in parts by weight set forth in the following TABLE 5.

                  TABLE 5                                                         ______________________________________                                        Composition          12A    12B                                               ______________________________________                                        Ultraviolet light absorber.sup.1                                                                    4.0    4.0                                              Methylamyl ketone    124.0  123.0                                             Colloidal silica dispersion.sup.2                                                                   18.4   0                                                Acrylic resin.sup.3  158.4  171.8                                             Crosslinking agent.sup.4                                                                           100.2  100.2                                             ______________________________________                                         .sup.1 As described in footnote 1 to Table 1.                                 .sup.2 The dispersion of colloidal silica in dipropyleneglycol                monomethylether as described in Example 2.                                    .sup.3 As described in footnote 11 to Table 1.                                .sup.4 A polymeric butylated melamineformaldehyde resin at about 67           percent by weight solids in a solvent mixture containing 22 percent by        weight butanol and 78 percent by weight naphtha and having a naphtha          tolerance ranging from 140 to 290.                                       

Next, samples of compositions 12A and 12B are each catalyzed with aphenyl acid phosphate solution and a sulfonic acid solution to formtopcoating compositions 12C and 12D respectively, as set forth in thefollowing TABLE 6. The percent by weight spray solids and the sprayviscosities for topcoating compositions 12C and 12D are also set forthin TABLE 6.

                  TABLE 6                                                         ______________________________________                                                          Topcoating                                                                            Topcoating                                                            Compo-  Compo-                                                                sition 12C                                                                            sition 12D                                          ______________________________________                                        Composition 12A     202.5     --                                              Composition 12B     --        200.0                                           Phenyl acid phosphate solution.sup.1                                                               1.0       1.0                                            Sulfonic acid solution.sup.2                                                                       0.73      0.73                                           No. 4 Ford Cup Viscosity in seconds                                                               28.9      29.1                                            Percent by weight spray solids                                                                    49.4      50.8                                            ______________________________________                                         .sup.1 Phenyl acid phosphate PA75 from Mobil Chemical Corp. which is          diluted to 50 percent by weight with isopropanol.                             .sup.2 Dinonylnaphthalene disulfonic acid available as NACURE 155 from        King Industries.                                                         

Next, unprimed steel panels are spray coated with a high-solidsthermosetting acrylic basecoating composition (available as DCT 15472from PPG INDUSTRIES, INC.) at a No. 4 Ford Cup viscosity of 19 secondsafter dilution with methylamyl ketone to form a basecoat. The basecoatis allowed to flash for 2 minutes at ambient atmospheric conditionsafter which topcoating compositions 12C and 12D are spray applied to thebasecoats to form topcoats. The resulting composite basecoat/topcoatfilms are cured at 285° F. (141° C.) for 30 minutes. Properties of theresulting cured films are as set forth in the following TABLE 7.

                  TABLE 7                                                         ______________________________________                                                        Example 12C                                                                            Example 12D                                          ______________________________________                                        Approximate dry film                                                                       Basecoat 0.6-0.7    0.6-0.7                                      thickness (mils)                                                                           Topcoat   1.4-1.55  1.3-1.4                                                   Total     2.25      2.0-2.1                                      20° Gloss      87         87                                           Definition of Image   60.5       55.0                                         (DOI)                                                                         Appearance            No sag     Sag is                                                             observed   apparent at                                                        even at panel                                                                            top and bottom                                                     edges      edges of panel                               ______________________________________                                    

Thus, Example 12C illustrates that the method of the invention can beutilized to control sagging of the high solids, clear topcoat and stillprovide a composite, cured film having a high degree of gloss.

What is claimed is:
 1. A method of coating a substrate comprising thesteps of:(A) coating a substrate with one or more applications of abasecoating composition comprising:(1) an organic film-forming resin,and where the film-forming resin can be crosslinked, optionally acrosslinking agent for the film-forming resin, (2) a solvent system forthe film-forming resin of the basecoating composition, (3) organicpolymeric microparticles which are insoluble in the solvent system ofthe basecoating composition and which have a diameter in the range offrom about 0.01 to about 40 microns, and (4) pigment particles to form abasecoat; and (B) coating the basecoat with one or more applications ofa topcoating composition comprising:(1) an organic film-forming resinwhich may be the same or different from the film-forming resin of thebasecoating composition, and where the film-forming resin of thetopcoating composition can be crosslinked, optionally a crosslinkingagent for the film-forming resin of the topcoating composition, (2)substantially colorless, substantially inorganic microparticles ofsilica stably dispersed in the topcoating composition ranging in sizefrom about 1 to about 150 nanometers wherein the silica has beenincorporated in an alcohol in the form of a stable colloidal dispersionof the silica in the alcohol, and wherein the inorganic microparticlesare present in the topcoating composition in an amount ranging fromabout 1 to about 20 percent by weight based on the weight of organicfilm-forming resin, optional crosslinking agent, and inorganicmicroparticles, and (3) a solvent system for the organic film-formingresin of the topcoating composition to form a transparent topcoat. 2.The method of claim 1 wherein the organic film-forming resin of thebasecoating composition comprises a crosslinkable resin having a weightaverage molecular weight of from about 500 to about 10,000 and theorganic film-forming resin of the topcoating composition comprises acrosslinkable resin having a weight average molecular weight of fromabout 500 to about 10,000.
 3. The method of claim 1 wherein prior toincorporation in the coating composition the substantially inorganicmicroparticles range in size of from about 1 to about 100 nanometers. 4.The method of claim 1 wherein prior to incorporation in the coatingcomposition the substantially inorganic microparticles range in sizefrom about 3.5 to about 50 nanometers.
 5. The method of claim 1 whereinthe silica has been treated by heating the silica in the presence of amonohydric alcohol, a polyol, or a mixture thereof at a temperature ofat least 180° C.
 6. The method of claim 1 wherein the basecoatingcomposition and the topcoating composition are applied by conventionalspraying to the substrate at a combined total solids content of at least50 percent by weight of the sum of the basecoating composition and thetopcoating composition.
 7. The method of claim 2 wherein the basecoatingcomposition contains a crosslinking agent for the crosslinkable resin ofthe basecoating composition and the topcoating composition contains acrosslinking agent for the crosslinkable resin of the topcoatingcomposition.
 8. The method of claim 1 wherein at least a portion of thepigment particles are metallic flakes.
 9. The method of claim 8 whereinthe metallic flakes comprise aluminum flakes.
 10. The method of claim 1wherein the topcoating composition additionally comprises organicpolymeric microparticles which are insoluble in the solvent system ofthe topcoating composition and which have a diameter in the range offrom about 0.01 to about 40 microns.
 11. The product produced by themethod of claim
 1. 12. The product produced by the method of claim 6.13. The product produced by the method of claim 8.